Although any micromechanical elements are applicable, the present invention and the problems underlying it are elucidated by referring to elements based on silicon.
Manufacturing methods for micromechanical sensor devices, for example, for micromechanical rotation rate sensors and acceleration sensors, are well-known in the related art.
As described, for example, in German Published Patent Application No. 195 37 814, a plurality of free-standing, thick polycrystalline functional structures are produced on a substrate. Buried printed conductors and electrodes are situated under these functional structures. The micromechanical functional structures manufactured in this way are sealed in the further process sequence using a cap wafer. Depending on the application, a suitable pressure is enclosed within the volume sealed by the cap wafer.
In the case of rotation rate sensors, a very low pressure is enclosed, typically 1 mbar. The background is that in the case of rotation rate sensors, a part of the movable structure is driven resonantly and the deflection resulting from the Coriolis force is measured. In the case of low pressure, it is possible to excite an oscillation very simply using relatively low voltages due to the low attenuation. The same applies also to magnetic field sensors which oscillate resonantly in an external magnetic field via a current, it being possible to determine the magnetic field via the deflection of the resonant oscillations.
In the case of acceleration sensors, it is, however, not desirable for the sensor to begin to oscillate, which would be possible if an external acceleration were applied. For that reason, these acceleration sensors are operated at high internal pressures, typically at 500 mbar. In addition, the surfaces of such acceleration sensors are also often provided with organic coatings, which prevent an adhesive bonding of the movable structures.
FIG. 4 shows a schematic cross-sectional view for elucidating an exemplary micromechanical sensor device and a corresponding manufacturing method for illustrating the problems underlying the present invention.
In FIG. 4, reference symbol M denotes an MEMS substrate, on which are deposited insulation layers 4a, 4b, for example, oxide layers, as well as a printed conductor layer 3 lying between them. Across layers 3, 4a, 4b is situated a micromechanical functional layer 5 made of polysilicon, which is partially anchored on insulation layer 4b and partially anchored on printed conductor layer 3 and as a result is electrically connected to the latter. An acceleration sensor device S1 and a rotation rate sensor device S2 are structured in micromechanical functional layer 5. Sensor devices S1, S2 are capped in separate hermetically isolated caverns KV1, KV2 by a cap wafer K via a bond connection B.
An exemplary electrical contacting of acceleration sensor device S1 is shown, which extends from micromechanical functional layer 5 via printed conductor layer 3 from acceleration sensor device S1 to a contact KO situated outside of cap wafer K.
Such combinations of an acceleration sensor device S1 and a rotation rate sensor device S2 may be designed to be very small and manufactured cost-effectively.
The different pressure which is needed in cavern KV 1 of acceleration sensor device S1 and cavern KV2 of rotation rate sensor device S2 may be achieved by using a getter layer G in cavern KV2 of rotation rate sensor device S2.
When cap wafer K is bonded to micromechanical functional layer 5 with the aid of bond connection B, a high pressure is initially enclosed in both caverns KV1, KV2, which is suitable for acceleration sensor device S1. Subsequently, the getter of getter layer G is activated via a temperature step. The getter pumps (getters) the volume of cavern KV2 of rotation rate sensor device S2 to a low pressure. In order to enclose a defined pressure in it, a mixed gas including an inert gas and a gas which is readily gettered, for example, N2, is used. N2 is gettered and a non-gettering inert gas of the mixed gas, for example, Ne or Ar, then defines the internal pressure in cavern KV2 of rotation rate sensor device S2. The internal pressure defined by the N2/Ne or N2/Ar gas mixture remains in cavern KV1 of acceleration sensor device S1.
Such a use of a getter layer, which is applied in the interior of a cavern of a cap wafer, which caps a micromechanical sensor device, is known from International Published Patent Application No. 2007/113325.
United States Published Patent Application No. 2013/0001710 describes a method and a system for forming an MEMS sensor device, a CMOS wafer being bonded to an MEMS wafer including a micromechanical sensor device. For example, an evaluation circuit for the micromechanical sensor device may be integrated into the CMOS wafer, the evaluation circuit being electrically contactable via the bond connection. In this example, the CMOS wafer assumes the function of the cap wafer.
Should combinations of acceleration sensor devices and rotation rate sensor devices or magnetic field sensor devices also be manufactured in such a system, it would be necessary to provide a getter layer on the CMOS wafer similar to the example according to FIG. 4.
Such a system having a getter layer on a CMOS wafer is, however, critical from two points of view.
Getter layers are normally sputtered on using a shadow mask. This method is very imprecise and, in order to sputter onto a small surface in a defined manner, very large local restrictions must be provided, with the result that the size of such sensor devices is increased unnecessarily.
The activated getter layer is moreover very reactive. If the movable micromechanical functional layer thus comes into contact with the sensor device in the case of a deflection, the micromechanical functional layer remains bonded to it. In other words, it may occur that the micromechanical sensor devices no longer function after a mechanical shock.